This invention relates to an exposure apparatus in which projection of a pattern of a first object onto a second object through an optical system and subsequent stepwise movement of the second object relative to the first object are repeated to thereby repeatedly print the pattern on the second object. More particularly, the invention relates to an exposure apparatus to be used in the manufacture of semiconductor devices such as integrated circuits (ICs), large scaled integrated circuits (LSIs), very large scaled integrated circuits (VLSIs), etc.
In the field of manufacture of semiconductor devices as mentioned above, exposure apparatuses for projecting, at a reduced magnification ratio, a circuit pattern of a reticle onto a wafer through an optical system to thereby print the reductively projected circuit pattern onto the wafer are known. An example is a step-and-repeat type exposure apparatus, called a stepper.
In such exposure apparatus, the circuit pattern printed onto the wafer through one exposure usually occupies only a small area on the wafer corresponding to one or more chips of the semiconductor devices. Therefore, in order to print the circuit pattern of the reticle onto all the sections of the wafer surface, it requires repetition of exposure accompanied with stepwise movement of the wafer relative to the reticle along a plane orthogonal to the optical axis of a projection optical system. Frequently, for each reticle, ten or more repetitions of exposure are necessary.
When, after a first exposure step comprising repetitions of exposure with a first reticle has been made relative to a wafer, a second exposure step with a second reticle is to be made to the same wafer, the circuit pattern formed on the second reticle must be accurately projected onto or overlaid on each of the circuit patterns which have already been printed on the wafer. In order to achieve this, it is desired to effect, for each of the exposures, alignment between the already-printed circuit pattern and the circuit pattern which is currently to be projected onto the wafer, namely alignment between the second reticle and the wafer. As an example of the technique for such alignment, a TTL (through-the-lens) type die-by-die alignment system is known. According to this alignment system, relative positional deviation between the reticle and wafer is detected through the projection optical system every time the stepwise movement of the wafer relative to the reticle is completed. When the relative positional deviation is detected, the positional relation between the reticle and wafer is corrected in accordance with the detected positional deviation and, thereafter, the exposure is effected. Such alignment process assures very precise alignment for each of the exposure operations. Therefore, a higher overlay accuracy for the circuit patterns of different reticles is advantageously attainable over the entire wafer surface.
Such alignment process however involves inconveniences as follows:
That is, according to such alignment process, plural alignment operations are required throughout the exposures of the entire wafer surface relative to the circuit pattern of one reticle. This results in a corresponding decrease in the throughput of the apparatus. Particularly, in the case of die-by-die alignment, plural alignment operations corresponding to the repetitions of exposures are required. It is therefore a significant problem to improve the throughput, in an exposure apparatus employing the alignment process as described above.